Narrow bandwidth mechanical filter using large area coupling wires

ABSTRACT

A mechanical filter of the stacked disc-type using a diameter mode disc. A first coupling wire means connects all discs together and specifically is connected to points on the diameter mode disc perimeter which vibrate in a first phase. A second coupling wire means connects all discs together and specifically is connected to the diameter mode disc perimeter at a point whose phase of vibration is 180* removed from said first phase. The difference between the cross-sectional area of said first and second coupling wire means is the effective cross-sectional area of the total coupling wire means. Since bandwidth is proportional to the effective cross-sectional area of the coupling means, the filter has the desired narrow bandwidth and also substantial mechanical strength.

United States Patent Dallas, Tex.

[54] NARROW BANDWIDTH MECHANICAL FILTER USING LARGE AREA COUPLING WIRES 1 Claim, 13 Drawing Figs.

[51] Int. Cl 1103b 9/26 [50] Field of Search 333/71, 72,

[56] References Cited UNITED STATES PATENTS 3,135,933 6/1964 Johnson 333/71 3,439,295 4/1969 Bise 333/71 3,440,572 4/1969 Bise.... 333/72 X 3,440,574 4/ 1969 Johnson et al. 333/72 3,488,608 1/1970 Johnson 333/71 3,516,029 6/1970 Johnson OTHER REFERENCES Johnson and Teske, A Mechanical Filter Having General Stopband Characteristics," IEEE Transactions On Sonics and Ultrasonics; July 1966, pp. 41- 48.

Primary Examiner-Eli Lieberman Assistant ExaminerMarvin Nussbaum Attorneys-Robert J. Crawford and Henry K. Woodward ABSTRACT: A mechanical filter of the stacked disc-type using a diameter mode disc. A first coupling wire means connects all discs together and specifically is connected to points on the diameter mode disc perimeter which vibrate in a first phase. A second coupling wire means connects all discs together and specifically is connected to the diameter mode disc perimeter at a point whose phase of vibration is 180. removed from said first phase. The difference between the cross-sectional area of said first and second coupling wire means is the effective cross-sectional area of the total coupling wire means. Since bandwidth is proportional to the effective cross-sectional area of the coupling means, the filter has the desired narrow bandwidth and also substantial mechanical strength.

PATENIEDJAII 4:972

SHEET 1 [IF 2 FIG. IA:

5 DIAMETER MODE 1 DIAMETER-I CIRCLE MODE 2 DIAMETER I CIRCLE MODE INVENTOR DONALD L. 52/85 4 DIAMETER MODE 5 ATTORNEYS PATENTEDJAN 4:972 3.633.133

m1 2 [IF 2 FIG 7 FIG 8 INVENTOR DONALD L. 3185 ATTORNEYS NARROW BANDWIDTI-I MECHANICAL FILTER USING LARGE AREA COUPLING WIRES This invention relates generally to mechanical filters and, more particularly, to mechanical filters having a narrow passband.

One of the more successful types of mechanical filters cur rently used in precision electronic gear comprises a plurality of discs stacked one upon the other with their axeslying along a common line and held in such position by coupling wires extending along the side of said stack and secured to the perimeters of the discs.

The discs employed in the mechanical filters havemany different vibration modes which occur at many frequencies occurring over a range extending from a few kHz. up to several hundred thousand kHz. The modes of vibration are of two principal types; one of which is known as the circle mode-type vibration and the other of which is known as the diameter mode-type vibration. In the circle mode-type vibration the nodal lines are circular in nature and concentric with the axis of the disc. In the diameter mode-type vibration the nodal lines lie along diameters of the disc. There can be multiple vibrations in both the circle and the diameter-type modes. For example, at a first frequency the disc might vibrate with a single circle nodal line and at a second frequency it might vibrate at two or three circular nodal lines. Similarly, the diameter mode-type vibrations can occur at one frequency with two diameter nodal lines positioned in quadrature, and at a different frequency with three diameter nodal lines and at still other frequencies with four and five diameter nodal lines. At still other frequencies, combinations of circle modes and diameter modes of vibration can occur simultaneously.

The modern mechanical filter, in many forms today, employs only circle mode-type vibrations in all of the discs of the filter. However, in some filters a combination of circle and diameter modes are employed.

Generally speaking, mechanical filters owe their success to their sharp passband characteristics curve and also to general reliability of operation. One limitation, however, of prior art mechanical filters is the obtaining of a narrow passband frequency response curve. The bandwidth is determined primarily by the cross-sectional area of the coupling wires, with a large bandwidth being produced when the total crosssectional area of the coupling wires is large and a narrower bandwidth being produced when the total cross-sectional area of the coupling wires is smaller. The use of coupling wires having a total small sectional area to produce narrow passbands raises serious mechanical problems. More specifically, if the total cross-sectional area of the coupling wires becomes too small, the mechanical filter will not have sufficient structural strength to withstand the stresses and strains it will undergo in many applications, such as for example, in airborne gear, or in any gear subject to accelerating forces.

A primary object of the present invention is a commercially feasible mechanical filter having anarrow bandwidth.

A second purpose of the invention is a mechanical filter having a narrow bandwidth and also having substantial structural strength.

A third purpose of the invention is to find a mechanical filter having a narrow bandwidth and a comparatively large total cross-sectional area of coupling wires.

A fourth object of the invention is the improvement of mechanical filters, generally.

In accordance with the invention there is provided a mechanical filter section comprised of a first and a second disc, with the first disc having a diameter mode-type vibration and the second disc having a circular mode-type vibration. A first coupling wire means is secured to the perimeters of the first and second discs and specifically is secured to the diameter modetype disc at points on the perimeter thereof all of which have the same phase of vibration. The second coupling wire means is also secured to the perimeters of said first and second discs, and specifically is secured to the diameter modetype disc at points on the perimeter thereof having a phase of vibration 180 removed from the phase of said first points. The

difference between the total cross-sectional area ofthe first coupling wire means and the cross-sectional area of the second coupling wire-means represent the efiectivecrosssectional area of the coupling wire means connecting together said first and second discs, since the first and second coupling wire means transfer energy which tend to cancel each other out.

In accordance with a feature of the invention, the said diameter mode disc is segmented in order to remove from the passband a spurious mode of vibration which, as will be discussed in detail in the specification, is created as a result of the loading effect produced on the diameter mode disc when said first and second coupling wires are connected to the perimeter thereof.

In accordance with another feature of the invention, many different types of diameter modes of operation can be employed in the present invention. The principal criteria of successful operation is that the first and second coupling wires connect together the pair of discs at points on the perimeter of said diameter mode disc whose vibrations are out of phase with each other.

In accordance with a third feature of the invention, several diameter mode-type discs can be used in a mechanical filter. For example, in a five-disc mechanical filter, the second and fourth discs can be diameter mode-type discs and the first, third, and fifth discs can be circle mode-type discs. The first and second coupling wire means are connected to the perimeters of all of said discs, and further are connected to the perimeters of the diameter mode discs at points whose vibrations are 180 out of phase with each other, as discussed above.

The above-mentioned and other objects and features of the invention will be more fully understood from the following detailed description thereof when read in conjunction with the drawings in which:

FIGS. lA-lB show two circle-diameter modes of vibration of a disc;

FIG. 2 is a perspective view of a five-disc mechanical filter constructed in accordance with the principles of the present invention;

FIG. 3 is a perspective view of a two-disc mechanical filter section comprising a diameter mode disc and a circle mode disc and which illustrates the basic principle of the invention;

FIG. 4 is a schematic diagram of the equivalent electrical circuit of the two diameter disc mechanical filter of FIG. 2;

FIGS. 5, 7, and 8 are circuit diagrams for transforming the circuit of FIG. 4 to the circuit of FIG. 6; the circuit of FIG. 6 illustrating the narrow passband obtained by the mechanical filter of the invention.

FIGS. 9A9D show additional modes of vibration which can be used in the invention.

The specification will be discussed in three separate sections as follows:

I. NATURE OF DIAMETER MODE DISCS-This section will discuss the characteristics of a diameter mode-type disc.

II. MECHANICAL FILTER SECTION AND EQUIVALENT ELECTRICAL ClRCUIT-This section describes the physical structure of the present invention and also the equivalent electrical circuit thereof.

III. CIRCUIT TRANSFORMATION-This section describes the circuit transformations required to transform the electrical equivalent circuit of the mechanical filter section into a circuit form whereby it can be readily observed that a narrow bandpass filter is obtained even though the coupling wires have a relatively large cross-sectional area.

I. NATURE OF DIAMETER MODE DISCS Referring now to FIG. 1A and FIG. 18 there are shown two modes of the same disc. More specifically, in FIG. 1A there is shown a combination of single circle-two diameter mode vibration of a disc 12. Such a vibration will occur in the size disc currently being used in most mechanical. filters in a frequency range of 400 to 500 kHz.

It is a characteristic of discs that when they are loaded, as by the attachment of coupling wires 20, 21, and 22, that a second circle-diameter mode of vibration will occur, as shown in FIG. 18. However, in FIG. IE it will be noted that the diameter nodal lines of vibration are shifted 45 with respect to the diameter nodal lines in FIG. 1A. Further, the frequency of vibration of the mode shown in FIG. 1B is about 7 kHz. less than that shown in FIG. 1A, with center frequencies of 415 kHz. and 422 kI-Iz., respectively, as shown.

The shaded portions of the discs of FIG. 1A and FIG. 1B represent vibrations in a first phase whereas the unshaded portions represent vibrations in a phase 180 removed therefrom. For example, in FIG. 1A the shaded portions 17 and 18 within the circle nodal line 19 vibrate 180 out of phase with the unshaded portions within said circular nodal line 19. Similarly, the shaded portions and 16 outside the circular nodal line 19 vibrate 180 out of phase with the unshaded portions outside circular nodal line 19. It is to be noted that all the shaded portions in FIG. 1A, both within and outside the circular nodal line 19, vibrate in phase, as do the shaded portions in FIG. 18.

II. MECHANICAL FILTER SECTION AND EQUIVALENT ELECTRICAL CIRCUIT In FIG. 2 there is shown a five-disc mechanical filter including three circle mode discs and two circle-diameter mode discs, arranged in alternate fashion. More specifically, discs 30, 32, and 34 are circle mode discs and discs 31 and 33 are segmented circle-diameter mode-type discs. Coupling wires 20" and 21" correspond to similarly identified coupling wires in FIG. 1 (unprimed) and, as can be seen in FIG. 2, are connected to all five discs and specifically are connected near the center of vibrating sections of diameter mode discs 31 and 33 which have the same phase of vibration.

The coupling wire 22" is also connected to all five discs and specifically is connected to segments 49 and 47 of diameter mode discs 31 and 33, which segments vibrate 180 out of phase with the segments to which coupling wires 20" and 21 are connected. Thus the energy transferred along coupling wires 21" and 20 is 180 out of phase with the energy transferred along coupling wire 22". Consequently, the two transferred energies tend to cancel each other.

The input to the mechanical filter of FIG. 2 is contained within the dotted block 37 and consists of an AC source 50, a current limiting resistor 39, a coil 41, and a transducer device such as a ferrite rod 41. The output means 38 of the mechanical filter comprises a ferrite transducer rod 44, a coil 43 wound thereon, and a suitable load 42.

Referring now to FIG. 3, there is shown a two-disc section of the five-disc filter of FIG. 2. More specifically, in FIG. 3 discs 33 and 34', corresponding to discs similarly identified in FIG. 2, are shown. The reason for showing a two-disc section is that the principle of the invention can more clearly be shown by employing the equivalent electrical circuit of a twodisc section. It is a relatively simple matter to expand the theory of an equivalent circuit of a two-disc circuit into that of a five-, six-, or seven-disc filter, as desired.

Referring now to FIG. 4, there is shown the equivalent electrical circuit of the two-disc filter of FIG. 3. In FIG. 4 the tank circuits 33" and 34" correspond, respectively, to discs 33 and 34 of FIG. 3. Inductor L corresponds to coupling wires 20" and 21" of FIG. 3, and inductor L, and the phase-inverting transformer T, correspond to the coupling wire 22" of FIG. 3. The phase-inverting transformer T, represents the fact that the coupling wire 22" of FIG. 3 is connected 0 segment 47' of disc 33, which segment vibrates in a phase 180 removed from the phase of vibration of the two segments 45' and 46 to which coupling wires 20" and 21 are connected.

III. CIRCUIT TRANSFORMATION In transforming the circuit of FIG. 4 to that of FIG. 6, it is necessary to utilize the basic transformation shown in FIG. 7 and FIG. 8. Specifically, the circuits of FIG. 8 and FIG. 7 are equal. It will be observed that in transforming from the circuit of FIG. 7 to that of FIG. 8, the value L and transformer T of FIG. 7 change, in FIG. 8, to a pie network with the series arm thereof having a value L and the two parallel branches each having a value L/2. Inverting 1:1 transformer T of FIG. 7 does not appear in FIG. 8.

Applying the above principle to FIG. 4, inductor L, and inverting 1:1 ratio transformer T, can be transformed into a circuit including, in FIG. 5, inductor L, and the two inductors L,/2. Combining inductors L,/2 with parallel inductors L and combining inductor L, with its parallel inductor L, of FIG. 5, the circuit of FIG. 6 is obtained.

In the expression near the circuit of FIG. 6, it can be seen that the value of L, approaches infinity as the value of L approaches the value of L, in FIG. 5. It is a characteristic of electrical filter theory that when inductor L in the circuit of FIG. 6 approaches infinity the bandwidth becomes increasingly narrow. Consequently, the circuit of FIG. 6 can represent a filter circuit with a narrow bandwidth. Further, since the circuit of FIG. 6 represents the electrical equivalent of the structure of FIG. 3, it can be seen that the mechanical filter structure of FIG. 3 can have a narrow bandwidth simply by making small the difference between the cross-sectional areas of the two coupling wires 20" and 21" and the coupling wire 22". Further, since the resultant equivalent inductance of the circuit of FIG. 3 is obtained by the difference in cross-sectional areas between coupling wire 22" and the two coupling wires 20" and 21, it can be seen that there is no restriction on the total cross-sectional area of the three coupling wires 20", 21", and 22" of FIG. 3. Thus the problem of structural strength is met while at the same time obtaining a filter section having a narrow passband.

In FIG. 9 there are shown some other modes of vibration, all of which include at least one diameter mode of operation and which can be utilized in the present invention. In each of the structures of FIG. 9, the coupling wires are shown connected to the disc in a manner which would provide for a cancellation of energy between certain ones of the coupling wires. For example, in FIG. 9A, coupling wires 60 and 61 are connected to segments of the disc which vibrate in a phase removed from those segments of the disc to which the coupling wires 62 and 63 are connected. By proper selection of the total crosssectional areas of coupling wires 60 and 61 as compared with the cross-sectional areas of coupling wires 62 and 63, the overall net effect can be a coupling of a small cross-sectional area and thus a narrow bandwidth.

Similarly, in the structure of the discs of FIGS. 9B, 9C, and 9D the difference in the cross-sectional areas of those coupling wires connected to the shaded portions of the disc and the cross-sectional areas of the coupling wires connected to the unshaded area portions of the disc, can be caused to be large or small, thus making the resultant bandwidth increasingly narrow as the difference becomes increasingly less.

It is to be understood that the forms of the invention shown and described herein are but preferred embodiments thereof and that various changes may be made in the arrangement of discs without departing from the spirit or scope thereof.

I claim:

1. A narrow bandwidth mechanical filter comprising:

a first plurality of circle mode-type discs;

a second plurality of diameter mode-type discs;

the discs of said first and second plurality of discs being positioned alternately and spaced apart with their axes lying along a common line;

first coupling wire means coupling points on the perimeters of said circle mode-type discs to points of the same phase on the perimeters of said diameter mode-type discs; second coupling wire means coupling points on the perimeters of said circle mode-type discs to points on the perimeters of said diameter mode-type discs having a phase of vibration 180 different from the phase of the other of said points on said diameter mode-type discs; the total cross-sectional area of said first coupling wire means being different from the total cross section area of said second coupling wire means. 

1. A narrow bandwidth mechanical filter comprising: a first plurality of circle mode-type discs; a second plurality of diameter mode-type discs; the discs of said first and second plurality of discs being positioned alternately and spaced apart with their axes lying along a common line; first coupling wire means coupling points on the perimeters of said circle mode-type discs to points of the same phase on the perimeters of said diameter mode-type discs; second coupling wire means coupling points on the perimeters of said circle mode-type discs to points on the perimeters of said diameter mode-type discs having a phase of vibration 180* different from the phase of the other of said points on said diameter mode-type discs; the total cross-sectional area of said first coupling wire means being different from the total cross section area of said second coupling wire means. 